The present invention relates generally to molding and, in particular to a vacuum press for molding multiple laminar products.
Electrical circuit boards are typically constructed by bonding a plurality of substrates or layers of material together to produce a rigid structure having desired electrical properties. Depending on the application, the individual substrates may be electrically insulating or electrically conductive. A conductive layer usually comprises a relatively thin film of conducting foil such as copper which may define an electrical circuit. Insulating layers are constructed from suitable dielectric or insulating materials.
Circuit boards can be manufactured using a batch process in which individual cut sheets of substrate are stacked one atop the other and are bonded together to form the unitary, rigid board. Batch processes are typically carried out using either a hydraulic press or a pressure vessel. In the first method, the layers of substrates are impregnated with a liquid resin composition or other material, often termed xe2x80x9cprepreg.xe2x80x9d The impregnated layers are then stacked, registered and placed within a pressure platen assembly forming part of the hydraulic press. The stack is then compressed by the platen assembly and heated to an elevated temperature. The combination of pressure and heat softens the prepreg and causes bonding between the individual layers. The high pressures and heat necessary to achieve bonding, if not carefully controlled, can cause problems in the finished product. For example, the high pressure can cause dimensional changes in the boards as well as misregistration between the layers. The heat and pressure if not uniform can also cause warping and twisting in the boards. It has been found that complex, multilayer applications cannot be readily produced using a conventional hydraulic press because an excessive scrap rate makes the process economically disadvantageous.
The use of an evacuated process chamber in which the circuit boards are bonded reduces the need for high pressure. U.S. Pat. No. 4,666,551 to Soberay et al. discloses a vacuum press that uses an evacuated process chamber having a hydraulic ram mounted below the platen assemblies and within the process chamber. While this press is commercially successful, it is believed that the placement of the hydraulic ram within the vacuum chamber introduces a risk of contaminating the chamber with hydraulic fluid and makes maintenance difficult. The height of the process chamber is dictated to some extent by the footprint of the hydraulic ram, so that it may be necessary to excavate below the press or have an operator stand on a platform to operate the press.
The present invention provides a new and improved vacuum press for bonding a plurality of laminar substrates that features an upward acting ram assembly that is mounted above the pressing platens to eliminate the need to provide a pit below operator height to accommodate a ram. According to the present invention, the ram assembly may be located outside the vacuum chamber to facilitate maintenance and minimize chamber contamination due to the presence of hydraulic fluid and/or oil associated with the ram.
A processing chamber having an interior portion defined by two side walls, front and rear walls, a chamber ceiling, and a chamber floor encloses a lower bolster member. An upper force transfer member is connected to the lower bolster member so that movement in the upper force transfer member produces concurrent movement in the lower bolster member. A stationary bolster member is maintained between the upper force transfer member and the lower bolster member such that distance between the upper force transfer member and the stationary bolster member can vary.
A ram or actuator is connected to the upper force transfer member. The actuator moves the upper force transfer member between a load position and a press position. A plurality of platens having an upper surface onto which the laminar substrate is placed and a lower force receiving surface are disposed within the processing chamber between the chamber ceiling and the lower bolster member. The platens are in predetermined alignment with the lower bolster member and as the lower bolster member is moved by the actuator toward the pressing position, the platens move into confronting engagement such that clamping forces are applied to the platens by the actuator.
In an exemplary embodiment, the stationary bolster member defines the ceiling of the chamber such that a lower surface of the stationary bolster member in located within the chamber and an upper surface of the stationary bolster member is located outside the chamber. The upper force transfer member is connected to the lower bolster member by a plurality of connecting rods and the connecting rods are in sealed sliding engagement with the stationary bolster member through apertures in the stationary bolster member. A hydraulic cylinder having an extendable rod substantially enclosed by a housing is mounted between the upper force transfer member and the stationary bolster member to move the upper force transfer member relative to the stationary bolster member.
According to a feature of the exemplary embodiment, each of the platens is suspended from the stationary bolster member on a set of hanger rods. The sets of hanger rods have staggered lengths so that an uppermost platen is suspended by a set of shortest hanger rods and a lowermost platen is suspended by a set of longest hanger rods. The hanger rods slidably protrude through the platens and engage a collar that limits downward movement of the platen and upward movement of the platen is allowed as the lower bolster member contacts the lowermost platen and slides the platens along their hanger rods toward the stationary bolster member. The lower bolster member has openings that allow the hanger rods to extend through the lower bolster as the lower bolster moves to the pressing position.
A seal assembly is disposed within the stationary bolster member aperture such that the assembly is accessible to outside the processing chamber. The seal assembly seals between the connecting rod and the stationary bolster member to inhibit vacuum leakage from the chamber as the connecting rods move therethrough.